The present invention relates to ion generators, and more particularly, to ion generators employed for electrostatic imaging.
A wide variety of techniques are commonly used to generate ions for electrostatic imaging. Conventional approaches include air gap breakdown, corona discharges, spark discharges, and others. The use of air gap breakdown requires close control of gap spacing, and typically results in nonuniform latent charge images. Corona discharges, which are widely favored in electrostatic copiers, provide limited currents and entail considerable maintenance efforts. Electrical spark discharge methods are unsuitable for applications requiring uniform ion currents, and provide limited service life. Other methods suffer comparable difficulties.
Apparatus and methods for generating ions representing a considerable advance over the above techinques are disclosed in commonly assigned U.S. Pat. No. 4,155,093, issued May 15, 1979. The ion generator of this invention, shown in one embodiment at 10 in FIG. 1, includes two conducting electrodes 12 and 13 separated by a solid insulator 11. When a high frequency electric field is applied between these electrodes by source 14, a pool of negative and positive ions is generated in the area of proximity of the edge of electrode 13 and the surface of dielectric 11. Thus, in FIG. 1, an air gap breakdown occurs relative to a region 11-r of dielectric 11, creating an ion pool in hole 13-h, which is formed in electrode 13. This air breakdown is characterized by a faint blue glow in the discharge region, and occurs at an inception voltage of around 400-600 volts. Such devices enjoy a self-limiting discharge characteristic, and enjoy extended and reliable service as compared with ion generators depending upon spark discharges.
The ions generated by these devices may be used, for example, to create an electrostatic latent image on a dielectric member 100 with a conducting backing layer 105. When a switch 18 is switched to position X and is grounded as shown, the electrode 13 is also at ground potential and little or no electric field is present in the region between the ion generator 10 and the dielectric member 100. However, when switch 18 is switched to position Y, the potential of the source 17 is applied to the electrode 13. This provides an accelerating electrostatic field between the ion reservoir 11-r and the backing electrode 16. Ions of a given polarity (in the generator of FIG. 1, negative ions) are extracted from the air gap breakdown region and charge the surface of the dielectric member 100. The charge formed on dielectric 100 is seen to increase generally in proportion to the number of excitation cycles of drive potential 14. Because it is necessary in order to form an electrostatic image on dielectric 100 to have a coincident drive voltage 14 and extraction voltage 17, this device is amenable to multiplexing.
One advantageous use of the ion generator disclosed in the above patent is for the formation of electrostatic images for high speed electrographic printing. When employed for this purpose, the apparatus of U.S. Pat. No. 4,155,093 encounters certain difficulties discussed in the Background of the Invention of the commonly assigned improvement patent, U.S. Pat. No. 4,160,257. With reference to the prior art sectional view of FIG. 2, the ion generator 20 includes in addition to the above-disclosed elements an apertured screen electrode 21, which is separated from the control electrode 13 and solid dielectric member 11 by a dielectric spacer 23. This additional electrode was found necessary to cure the problem of accidental erasure of a latent electrostatic image previously formed on the dielectric surface 100. This would occur in the apparatus of FIG. 1 if a high voltage alternating potential were imposed between the control and driver electrodes, without any extraction potential applied to the control electrode 13. In this instance, any previously formed charge image on the dielectric surface 100 would create an electrostatic extraction field tending to attract ions of opposite polarity from the control aperture 13-h, thereby partially or completely erasing the electrostatic image. As discussed in detail in U.S. Pat. No. 4,160,257, the inclusion of screen electrode 21 has been found to prevent such accidental image erasure by imposing a screen potential 28 between the screen electrode 21 and counterelectrode 105 of the same polarity as control potential 17.
Although the apparatus of U.S. Pat. No. 4,160,257 allows a fair degree of control over the size and shape of electrostatic images formed thereby, it suffers certain shortcomings. This is particularly true as respects the placement of the image. As is well known in the various printing technologies which rely on dot matrix imaging, it is highly advantageous to enhance the precision of locating the image elements, i.e. resolution. During the normal operation of U.S. Pat. No. 4,160,257, the image raster is defined by the length of the ion generator and the number of drive and control lines. Typical figures for these parameters are 20 drive lines, 128 control lines and an ion generator extent of 8.53 inches, which represents a resolution of approximately 300 dots per inch. Although this image density has been found reasonably satisfactory, it would be advantageous to increase the dot density beyond the limitations imposed by imaging speed. By increasing the density of the image raster, a commensurate improvement is achieved in the image quality range of this electrostatic imaging system.
Accordingly, it is a primary object of the invention to provide improved ion generating devices for the formation of electrostatic images. A principal related object is to improve the imaging capabilities of such systems while increasing the efficiency thereof.
Another object of the invention is to simplify the requirements of the driving electronics for such systems. This plays an important practical role, by reducing the cost of these systems.
A further object is to broaden the imaging capabilities of such electrostatic imaging systems. Specifically, it is desirable to be able to provide a variety of character fonts as well as a broadened tonal range.